(1) Field of the Invention
The invention relates to a regulated voltage supply circuit, and more particularly, to a regulated voltage supply circuit with noise compensation to optimize the power supply rejection ratio (PSRR).
(2) Description of the Prior Art
Regulated voltage supply circuits are used to create controlled voltage supplies for electronic systems. A common problem, which must be overcome in a voltage regulator circuit, is the coupling of noise from the power supply onto the regulated voltage line. This power supply noise can come from a variety of sources including digital and analog switching or external coupling. If this noise is carried in the regulated supply, then it will affect all of the circuits connected to this supply. This supply noise is especially problematic for sensitive analog circuits such as analog-to-digital converters and linear signal processing circuits.
A measure of the amount of power supply noise that is coupled onto the regulated supply is the power supply rejection ratio (PSRR). PSRR is calculated by dividing the ac noise signal present on the regulated dc voltage by the ac noise signal present on the unregulated power supply. It is typically desired that the PSRR be made as low as possible within the other design constraints for the regulator circuit.
Referring now to FIG. 1, an exemplary prior art method for optimizing PSRR is shown. A voltage follower circuit 58 comprising an amplifier is used to generate a regulated supply VOUT 50. The non-inverting input of this amplifier 58 is coupled to a reference voltage VREF 40 through the reference resistance RREF 54. The amplifier 58 is configured to voltage follow the reference voltage VREF 40 onto the output VOUT 50 while providing a low impedance output capable of driving all of the circuits for the regulated supply VOUT 50. The power supply, in this case is a battery, VBATT 30. To improve the PSRR performance, VBATT 30 is coupled to a low drop voltage regulator, or LDO, circuit 70.
The LDO circuit 70 and the capacitor 62 filter are used to create a filtered version VBF 66 of the power supply VBATT 30. Much of the power supply noise is filtered away before VBF 66 couples to the amplifier 58. Therefore, the PSRR response of the regulator circuit is improved. The prior art example has the disadvantage of the size and cost of the LDO circuit 70 and the capacitor 62. In addition, noise that is not filtered by the LDO and the capacitor is coupled directly onto the regulator amplifier 58.
Several prior art inventions describe power supply regulators and PSRR. U.S. Pat. No. 6,157,180 to Kuo describes a power supply regulator circuit with increased supply noise rejection ratio. The regulator uses a voltage follower circuit to buffer a reference voltage. The circuit topography eliminates compensation capacitance to thereby enhance the rejection ratio. U.S. Pat. No. 5,686,821 to Brokaw describes a single-loop voltage regulator. U.S. Pat. No. 5,910,886 to Coleman discloses a phase-shift power supply for use in a plasma deposition apparatus.
A principal object of the present invention is to provide an effective and very manufacturable regulated voltage supply circuit.
A further object of the present invention is to provide a regulated voltage supply circuit with optimized PSRR.
A still further object of the present invention is to optimize PSRR by a noise compensation circuit where noise on the power supply is phase shifted by 180 degrees and then added to the voltage reference of the regulator.
Another still further object of the present invention is to make the noise compensation circuit gain adjustable.
Another still further object of the present invention is to adjust the noise compensation gain by modulating noise on the power supply, measuring the noise on the regulated supply, and adjusting the gain to optimize the PSRR.
Yet another still further object of the present invention is to make the noise compensation circuit adjustable using an adjustable value resistor.
In accordance with the objects of this invention, a regulated voltage supply circuit having improved power supply rejection ratio is achieved. The circuit comprises, first, a voltage follower having an input, an output, and a power supply voltage. The input is coupled to a reference voltage. The output comprises the regulated voltage supply. Second, a means of compensating noise on the power supply voltage comprises phase shifting the power supply voltage 180 degrees and feeding back the phase shifted power supply voltage to the voltage follower input to thereby improve power supply rejection ratio. The means of compensating noise may comprise an adjustable gain. This adjustable gain may further comprise an adjustable value resistance. The adjustable gain is used in a to optimize the PSRR by testing comprising modulating noise on the power supply voltage, measuring the noise on the regulated voltage supply, and adjusting the gain.